The present invention relates to a method of purifying organic diphosphite compounds.
Organic diphosphite compounds have found extremely widespread use, for example as chelating ligands in homogeneous catalysis, and also as flame retardants, UV stabilizers, etc. Particular rhodium complexes with organic diphosphite compounds have been found to be useful as catalysts for the hydroformylation of olefins since they firstly have a high catalytic activity and secondly lead predominantly to linear aldehydes which are preferred for many applications. Organic diphosphite compounds are also suitable as ligands for transition metal complex catalysts for hydrocyanation, hydrogenation, carbonylation, hydroacylation, hydroamidation, hydroesterification, hydrosilylation, hydroboration, alcoholysis, isomerization, allylic alkylation or hydroalkylation.
Such diphosphite compounds, their preparation and their use as ligands in a hydroformylation process are described, for example, in EP 0 214 622 A2, U.S. Pat. No. 4,668,651, U.S. Pat. No. 4,748,261, U.S. Pat. No. 4,769,498, U.S. Pat. No. 4,885,401, U.S. Pat. No. 5,235,113, U.S. Pat. No. 5,391,801, U.S. Pat. No. 5,663,403, U.S. Pat. No. 5,728,861 and U.S. Pat. No. 6,172,267. The use in a hydrocyanation process is also described in U.S. Pat. No. 6,127,567.
Organic diphosphites of the general formula (A) are usually prepared by a process comprising the following steps:                a) reaction of a compound of the formula (A1) (=first aromatic diol) with phosphorus trichloride to give the phosphomonochloridite (A2)        
                b) reaction of the phosphomonochloridite (A2) with a compound of the formula (A3) (=second aromatic diol) to give the chelating diphosphite (A)        

The groups derived from the first aromatic diol (A1) in the organic diphosphites will hereinafter also be referred to as “side wings”.
The preparation of diphosphites in which at least one of the phosphorus atoms is not part of a heterocycle is carried out analogously by, in step a), reacting PCl3 with two molar equivalents of an appropriate monoalcohol instead of one molar equivalent of the first aromatic diol (A1). To prepare diphosphites in which the two phosphorus atoms are bridged by other groups, other diols can be used instead of the backbone diol (A3).
One possible way of removing the hydrogen halides liberated in the condensation reaction is the use of an at least stoichiometric amount of base, with nitrogen bases frequently being used. However, the removal of the resulting acid salts is frequently difficult and the salts can often not be recycled sensibly and have to be disposed of, which is associated with additional costs.
WO 2003/062171 and WO 2003/062251 describe a method of separating acids from reaction mixtures by means of an auxiliary base which with the acid forms a salt which is liquid at temperatures at which the desired product is not significantly decomposed during removal of the liquid salt and the salt of the auxiliary base forms two immiscible liquid phases with the desired product or the solution of the desired product in a suitable solvent. In other words, the acid salts of the auxiliary base behave like ionic liquids which are essentially immiscible with the actual reaction solvent. Preferred auxiliary bases of this type are 1-methylimidazole, 1-n-butylimidazole, 2-methylpyridine and 2-ethylpyridine. The methods described in WO 2003/062171 and WO 2003/062251 are suitable, inter alia, for phosphorylation reactions such as the above-described synthesis of phosphomonochloridites and the reaction thereof with an aromatic diol to give a diphosphite compound.
In general, organic diphosphite compounds have to be subjected after the synthesis to a purification in order to remove interfering impurities before use in a catalysis process.
Potential impurities can be independent of the synthesis process used, e.g. decomposition or other subsequent products typical of this class of material or be formed during the course of the synthesis. Problematical impurities are, firstly, impurities which can form complexes with transition metals such as rhodium, e.g. acetonitrile, and thus have a potential influence on use of the diphosphite compounds as catalysts. These include, for example, secondary organophosphites which will be discussed in more detail below. Also problematical are impurities which make the use of expensive apparatuses necessary, e.g. corrosive halides, especially chloride. Chloride ions are also known catalyst poisons for rhodium complex catalysts.
Adverse effects of impurities in the organic diphosphite compounds can affect the process itself in which they are used as ligands. Thus, impurities which act as catalyst poisons and/or lead to decomposition of the catalyst have an adverse effect on the catalyst operating life, which can over time lead to operational malfunctions. This applies, in particular, to the use of the organic diphosphite compounds in a continuous process in which impurities can accumulate. Adverse effects of these impurities can also affect the desired products produced in the respective process by having adverse effects on product properties, e.g. the storage behavior, the handleability, the odor, the color, the keeping qualities, etc.
The substantially complete removal of impurities is therefore a critical prerequisite for the organic diphosphite compound to be able to be used successfully in an industrial process.
Typical impurities from the synthesis of organic diphosphite compounds are residues of the base (generally an organic nitrogen-comprising compound, e.g. an amine) used for scavenging the hydrogen halide (generally HCl) liberated in the reaction, the acid salts of this base and possibly also residues of the hydrogen halide. Typical impurities from the synthesis also include catalysts which are intended to accelerate the reaction of the phosphorus trihalide with the aromatic alcohols. Even when, as described in WO 2003/062171 and WO 2003/062251, a compound whose acid salts behave like ionic liquids which are essentially immiscible with the solution of the organic diphosphite compound in an appropriate organic solvent, e.g. toluene, and can thus easily be separated off by phase separation is used as base, purification of the crude ligand solution is generally nevertheless absolutely necessary.
DE 103 60 771A1 teaches carrying out the reaction of phosphorus halides with organic compounds which have at least one OH group in the presence of a basic ion-exchange resin.
WO 2009/120210 and the US patent 2009/0247790 of the same priority date describe a process for preparing phosphomonochloridites which can be used as intermediate for introducing the side wings in the preparation of chelating diphosphite compounds. According to these documents, the reaction of PCl3 with an aromatic diol occurs in a solution comprising less than 5 mol % of a nitrogen base, based on mol of aromatic diol, with HCl formed being driven from the reaction solution and the reaction being carried out under essentially isothermal conditions. However, this is associated with the disadvantage that hydrogen chloride gas discharged as offgas stream has to be isolated in a separate scrubber and disposed of. In addition, solvent is generally also discharged with the offgas stream. However, in order to avoid emissions, the solvent entrained in the offgas has to be removed, which can be effected, for example, by incineration and requires an additional outlay.
WO 2010/042313 describes a process for preparing organic diphosphites, in which the reaction of PCl3 with the first aromatic diol forming the side wings is carried out in the presence of the second aromatic diol which bridges the two phosphorus atoms and the reactants are brought into contact with one another as a slurry in an organic solvent and the slurry comprises less than 5 mol % of a nitrogen base, based on mol of first diol, and the organic solvent has only a slight solvent capacity for HCl. This procedure leads to a reduction in the amount of acid salts formed by scavenging of the HCl by means of base in the condensation reaction.
Once again, the hydrogen chloride gas discharged as offgas stream has to be isolated and disposed of.
WO 2010/052090 and WO 2010/052091 describe processes for preparing 6-chlorodibenzo[d,f][1,3,2]dioxaphosphepin, which can be used as intermediate for introduction of the side wings in the preparation of chelating diphosphite compounds. In these processes, 2,2′-dihydroxybiphenyl suspended in an inert solvent is added to an excess of phosphorus trichloride under inert gas in a reactor with stirring and the gases formed are discharged from the reaction mixture. Thus, an addition of base in the reaction can be dispensed with. The hydrogen chloride discharged as offgas stream has to be collected, for which, according to the teachings of this document, a separate scrubber is used. However, to avoid emissions, the solvent entrained in the offgas has to be removed, which can, for example, be carried out by incineration and requires an additional outlay.
Further impurities which can be comprised in the solution of the crude organic diphosphite are the monoxide (B1) and dioxide (B2) thereof or the hemiligand (B3) formed by incomplete reaction of the backbone.

In Chem. Eur. J. 2011, 17, 2120, A. Christiansen et al. describe the formation of heteroatom-substituted secondary phosphine oxides as decomposition products and preligands in rhodium-catalyzed hydroformylation. The corresponding secondary organophosphites (C1) formed in the hydrolysis of tertiary phosphites represent a problematic impurity in the crude chelating diphosphite solution since they act as acid and decompose the acid-labile chelating diphosphites over the course of time. In addition, the compounds (C1) act as catalyst poison by complexing transition metals such as rhodium and when they accumulate in the reactor over prolonged periods of time can lead to deposition of the transition metal from the homogeneous reaction solutions and lead to rhodium losses. Since the transition metal is then no longer available for catalysis, operational malfunctions are the result. Especially in hydroformylation, the compounds (C1) can condense with the aldehydes formed to give •-hydroxyphosphonates (C2). Both the compounds (C1) and the compounds (C2) lead, as a result of their acidity, to hydrolytic decomposition of the chelating phosphite ligands. This process also proceeds autocatalytically since further (C1) is formed in the hydrolysis of the chelating phosphite ligands.

EP 0 285 136 A2 describes a method of purifying tertiary organophosphites by separating off secondary organophosphites, especially secondary organophosphites having a tetracoordinated phosphorus atom as in C1. This document refers to the problem that secondary organophosphites generally cannot be separated off from tertiary organophosphites by simple recrystallization since these compounds frequently cocrystallize. EP 0 285 136 A2 therefore teaches adding water and a Lewis base which selectively converts secondary organophosphites into salts of primary organophosphites to a solution of the secondary and tertiary organophosphites in an organic solvent so that the salts of primary organophosphites can then be separated off from the tertiary organophosphites. Suitable Lewis bases are NaOH and tertiary amines, e.g. triethylamine.
CN 101684130A describes a process for preparing chelating phosphites, in which                a.) the phosphomonochloridite forming the side wings is dissolved in dichloromethane,        b.) the aromatic diol which bridges the two phosphorus atoms is dissolved in triethylamine or a triethylamine/dichloromethane mixture,        c.) the solutions from a.) and b.) are mixed and reacted at from −40° C. to 20° C.,        d.) the resulting solution is stirred at from 20 to 30° C. for from 10 to 20 hours and        e.) deionized water is added to the solution from step d.), the mixture is stirred, the phases are allowed to separate, with the lower organic phase comprising the phosphite product.        
The chelating phosphites obtained in this way are characterized, inter alia, by a chloride ion content of less than 0.01% by weight (100 ppm).
US 2003/0100787 describes a process for preparing sterically hindered triaryl monophosphites, but a possible use for preparing diphosphites is not described. According to the preparative examples, the synthesis of these monophosphites is carried out by reaction of substituted phenols with PCl3 in the presence of pyridine and methylene chloride as solvent. After the reaction, the methylene chloride is distilled off and the monophosphite is induced to crystallize by addition of isopropanol.
Studies on the rhodium-catalyzed hydroformylation of 1-octene and styrene using bulky chelating phosphite ligands are described in Organometallics 1996, 15(2), 835-847. In the preparation of ligand (9) (6,6′-[[3,3′,5,5′-tetrakis(1,1-dimethylethyl)-[1,1′-biphenyl]-2,2′-diyl]bis(oxy)]bisdibenzo[d,f][1,3,2]dioxaphosphepin), it is stated that the ligand obtained after taking off the solvent and excess pyridine is firstly induced to crystallize by addition of acetonitrile and is then recrystallized from a toluene/acetonitrile mixture.
U.S. Pat. No. 5,312,996 describes, in column 18, line 60 ff., a ligand synthesis by reaction of 1,1′-biphenyl-3,3′-di-tert-butyl-5,5′-di-tert-butoxy-2,2′-diol with biphenol chloridite in toluene and in the presence of pyridine. The pyridinium chloride formed is filtered off from the reaction product obtained. The resulting solution is evaporated on a rotary evaporator until it has a syrupy consistency and the diphosphite obtained is then precipitated by addition of acetonitrile. The solid obtained is filtered off, washed with acetonitrile and dried.
It is an object of the present invention to provide a simple and effective method of purifying organic diphosphite compounds. The diphosphite compound obtained should have a purity which makes it possible for the diphosphite compound to be used as ligand in a continuous industrial process. Contamination with compounds from the production process, e.g. acetonitrile, which have an adverse effect on use of the organic diphosphites as ligands for catalysts for homogeneous catalysis should be avoided. In particular, the content of secondary organophosphites should also be very low. The organic diphosphite compound obtained should preferably be obtained in a solid form with good use properties. Such forms include, for example, crystals which are large enough for them to be able to be separated off readily by filtration and/or have only a small level of occlusions of solvent (occluded solvent) with impurities comprised therein.
It has now surprisingly been found that a crude organic diphosphite which is at least partly dissolved in an organic solvent can be effectively freed of the abovementioned impurities by precipitation by means of a precipitant (i.e. a solvent in which it is sparingly soluble).